1. Field of the Invention
The present invention relates to a laser light source device for obtaining a stable and high-output laser light by combining a fiber laser and a wavelength conversion element, and an image display device using such a laser light source device.
2. Description of the Background Art
High-output visible light sources having strong monochromaticity are necessary in realizing large-scale displays, high-luminance displays and the like. Out of three primary colors of red, green and blue, a red high-output semiconductor laser used in a DVD recorder or the like can be utilized as a small-scale light source having high productivity for a red color. However, for a green or blue light source, realization by means of a semiconductor laser or the like is difficult and there is a demand for a small-scale light source having high productivity.
For such a light source, a wavelength conversion device constructed by combining a fiber laser and a wavelength conversion element is realized as a low-output visible light source. Green and blue small-scale light sources are well known which use a semiconductor laser as an excitation light source for exciting the fiber laser and a nonlinear optical crystal as the wavelength conversion element.
On a laser display utilizing such high-output laser light sources, vivid images with high color purity can be displayed by utilizing laser light sources having suitable wavelengths since lights of the respective red, green and blue light sources are monochromatic lights. Such laser light sources enable the miniaturization of light sources by the use of lasers and the miniaturization of optical systems because lights can be easily focused, whereby palmtop image display devices can be realized.
On the other hand, the use of a laser having high coherency causes coherence noise called speckle noise to be formed in a displayed image. Accordingly, there have been proposed methods for removing speckle noise according to which the speckle noise is reduced by differentiating light paths depending on polarization directions using a prism (Japanese Unexamined Patent Publication No. 2004-151133); the wavefront of a light irradiated to a screen is made random by swinging an optical component to change a light path of a light source (Japanese Unexamined Patent Publication No. 2004-138669); a spectrum is caused to have a sideband using a light modulator to broaden the apparent light spectrum (Japanese Unexamined Patent Publication No. H09-121069); an oscillation wavelength is operated using the injection seeding technology to a solid-state laser (Japanese Unexamined Patent Publication No. H10-294517); and a semiconductor laser having a plurality of wavelengths is used as a module (Japanese Unexamined Patent Publication No. 2004-144794).
On the other hand, there have been proposed a fiber laser using a Raman fiber (WO 01/54238) and a semiconductor laser using a reflection element called a sampled grating which is a diffraction grating having a plurality of reflection wavelengths to realize a semiconductor laser capable of simultaneous multiwavelength oscillation (Japanese Patent No. 3689483). The construction of the fiber laser disclosed in WO 01/54238 is shown in FIG. 26A. This laser includes a sampled grating 2001, a Raman fiber 2002 and a broadband dielectric mirror 2003. By exciting the Raman fiber 2002 with an excitation light 2004, a light of λ1 is generated by the resonance between one reflection peak of the sampled grating 2001 and the broadband dielectric mirror 2003. Using this light of λ1 as an excitation light, a light of λ2 is generated by the resonance between another reflection peak of the sampled grating 2001 and the broadband dielectric mirror 2003. In this way, lights of λ3, λ4 are successively generated and a light 2005 of λ5 is finally extracted.
The construction disclosed in Japanese Patent No. 3689483 is shown in FIG. 26B. The combined construction of a Raman fiber and a sampled grating in FIG. 26B aims to generate an excitation light of a desired wavelength by a Raman shift. An emission area 2007 and a reflection area 2008 are formed on a semiconductor substrate 2006, wherein the reflection area 2008 has a sampled grating structure so as to be capable of simultaneous oscillation at a plurality of wavelengths. Control means (refractive index changing electrode) 2009 for changing the refractive index of the reflection area 2008 is provided in the reflection area 2008, so that the wavelength of the reflection area 2008 can be shifted.
U.S. Pat. No. 6,432,736 discloses an example of a semiconductor laser array in which two reflection areas having a sampled grating structure are provided to switch an oscillation wavelength. Further, U.S. Pat. No. 6,597,711 discloses an example in which the construction of U.S. Pat. No. 6,432,736 is applied to a fiber laser.
However, if the aforementioned prior arts are applied to a method for reducing speckle noise by using broadband light sources or light sources having a plurality of oscillation wavelengths, there have been problems of a higher cost, an enlarged device size and the like since a plurality of laser light sources are required. The semiconductor laser and the fiber laser light source using the sampled grating cannot obtain as high outputs as can be used as light sources for laser displays and, in addition, determine the oscillation wavelength by controlling the two reflection areas. Therefore, there has been a problem of complicating a control method for simultaneously controlling the oscillation wavelength and the laser output. There is another problem that a wavelength change by the control of the reflection areas is sensitive to external temperature. For the example of the fiber laser light source, a large stress needs to be applied to the fiber grating in order to obtain a large variable range, making the breakage of the fiber possible, wherefore there is a problem in the reliability of the fiber. There is an additional problem that the wavelength dependency of a gain of a laser medium triggers an oscillation output change in the case of switching the wavelength.
On the other hand, the aforementioned laser light source having a plurality of oscillation wavelengths has started being used in the medical field. Laser lights of different wavelengths are needed depending on treatments in the medical field. The wavelengths of laser lights particularly used in eye clinics are in the neighborhood of 530 nm, in the neighborhood of 600 nm and in the neighborhood of 1 μm. Laser lights in the neighborhood of 530 nm are used for the retinal coagulation of eyes, those in the neighborhood of 600 nm for the stoppage of fundal hemorrhage, and those in the neighborhood of 1 μm for cataract surgeries. Developments on laser light sources used for such ophthalmic treatments are being advanced at present and, particularly, there has been a need for a laser application device capable of obtaining laser lights of many wavelengths so as to cope with many treatments by one laser light source. Japanese Unexamined Patent Publication No. 2006-122081 discloses a laser device capable of multiwavelength oscillation by shifting an oscillation wavelength toward a longer wavelength side using a Raman fiber. Besides, lasers and the like have been developed which realize multiwavelength oscillation by utilizing a plurality of fluorescence peaks of a solid-state laser.
However, the above laser device capable of multiwavelength oscillation using the Raman fiber cannot simultaneously generate lights of two wavelengths. Further, since the gain of the Raman fiber laser is greatest at a wavelength of 1000 to 1100 nm, in the case of generating a light of 1100 to 1200 nm necessary for an orange light, a light having a broad emission spectrum of 1000 to 1100 nm is also generated. This has caused a problem of breaking a laser oscillator by the pulse oscillation. On the other hand, in the aforementioned laser capable of multiwavelength oscillation using the solid-state laser, the optical systems need to be switched for each oscillation wavelength, which has caused a problem of difficulty to switch to a desired wavelength in a moment. Further, since the property of the laser light largely differs at each oscillation wavelength, there has been a problem that obtained maximum outputs cannot be constant.
Further, in the construction of the conventional laser light source for medical use, a laser light generated by the laser oscillator has been propagated to a surgical handpiece by means of a hollow fiber or the like after having a fundamental wave thereof wavelength-converted into a visible light by a wavelength conversion element. However, about 30% of the visible light is lost to reduce propagation efficiency due to the coupling loss of the visible light from the laser oscillator to the hollow fiber and a propagation loss in the fiber. There has been also a problem that the handpiece is difficult to handle due to the handling of the fiber.